1. Field of the Invention
This invention relates generally to devices that emit electromagnetic radiation and, in particular, to a laser assembly comprising an optical fiber coupled to a semiconductor laser having a bottom laser cavity mirror and a top cavity mirror.
2. Description of the Related Art
The following descriptions and examples are not admitted to be prior art by virtue of their inclusion within this section.
Lasers have a wide range of industrial and scientific uses. There are several types of lasers, including gas lasers, solid-state lasers, liquid (dye) lasers, and free electron lasers. Semiconductor lasers are also in use. In semiconductor lasers, electromagnetic waves are amplified in a semiconductor superlattice structure. Semiconductor lasers may be diode lasers (bipolar) or non-diode lasers such as quantum cascade (QC) lasers (unipolar). Semiconductor lasers are used for a variety of applications and can be built with different structures and semiconductor materials, such as gallium arsenide (GaAs).
The use of semiconductor lasers for forming a source of optical energy is attractive for a number of reasons. Semiconductor lasers have a relatively small volume and consume a small amount of power as compared to conventional laser devices. Further, semiconductor lasers can be fabricated as monolithic devices, which do not require a combination of a resonant cavity with external mirrors and other structures to generate a coherent output laser beam.
A semiconductor laser typically comprises an active (optical gain) region sandwiched between two mirrors, one of which is serves as the exit mirror. The area between the reflective planes is often referred to as the resonator, or the Fabry-Perot resonance cavity in some cases. When the active region is pumped with an appropriate pumping energy, it produces photons, some of which resonate and build up to form coherent light in the resonant cavity formed by the two mirrors. A portion of the coherent light built up in the resonating cavity formed by the active region and top and bottom mirrors passes through the exit mirror as the output laser beam.
Various forms of pumping energy may be utilized to cause the active region to begin to emit photons. For example, semiconductor lasers of various types may be electrically pumped (EP) (by a DC or alternating current), or pumped in other ways, such as by optically pumping (OP) or electron beam pumping. EP semiconductor lasers are typically powered by applying an electrical potential difference across the active region, which causes a current to flow therein. Electrons in the active region attain high energy states as a result of the potential applied. When the electrons spontaneously drop in energy state, photons are produced (to carry away the energy lost by the transition, so as to conserve energy). Some of those photons travel in a direction perpendicular to the reflectors of the laser. As a result of the ensuing reflections, the photons can travel through the active region multiple times.
Stimulated emission occurs when an electron is in a higher energy level and a photon with an energy nearly equal to the difference between the electron""s energy and a lower energy interacts with the electron. In this case, the photon may stimulate the electron to fall into the lower energy state, thereby emitting a photon. The emitted photon will have the same energy as the original photon, and, if viewed as waves, there will be two waves emitted (from the electron""s atom) in phase with the same frequency. Thus, when the photons produced by spontaneous electron transition photons interact with other high energy state electrons, stimulated emission can occur so that two photons with identical characteristics are present. If a sufficient number of the electrons encountered by the photons are in the high energy state, the number of photons traveling between the reflectors tends to increase, giving rise to amplification of light, and thus lasing. The result is that coherent light builds up in the resonant cavity formed by the two mirrors, a portion of which passes through the exit mirror as the output laser beam.
Semiconductor lasers may be edge-emitting lasers or surface-emitting lasers (SELs). Edge-emitting semiconductor lasers output their radiation parallel to the wafer surface, while in SELs, the radiation output is perpendicular to the wafer surface. One type of SEL is the vertical-cavity surface-emitting laser (VCSEL). The xe2x80x9cverticalxe2x80x9d direction in a VCSEL is the direction perpendicular to the plane of the substrate on which the constituent layers are deposited or epitaxially grown, with xe2x80x9cupxe2x80x9d being typically defined as the direction of epitaxial growth an also the direction of emission of the output laser beam from the xe2x80x9ctopxe2x80x9d, exit mirror. Both EP and OP VCSEL designs are possible. VCSELs can have many attractive features compared to edge-emitting lasers, such as low threshold current, single longitudinal mode, a circular output beam profile, a smaller divergence angle, and scalability to monolithic laser arrays. The shorter cavity resonator of the VCSEL provides for better longitudinal mode selectivity, and hence narrower linewidths. Additionally, because the output is perpendicular to the wafer surface, it is possible to test fabricated VCSELs on the wafer before extensive packaging is done, in contrast to edge-emitting lasers, which must be cut from the wafer and at least partially packaged to test the laser. Also, because the cavity resonator of the VCSEL is perpendicular to the layers, there is no need for the cleaving operation common to edge-emitting lasers.
The VCSEL structure usually consists of an active (optical gain) region sandwiched between two mirrors (reflectors), such as distributed Bragg reflector (DBR) mirrors. The two mirrors may be referred to as the top (exit) DBR and the bottom DBR. Other types of VCSELs sandwich the active region between metal mirrors.
A VCSEL must have a means of pumping the active region to achieve gain. For example, in an EP VCSEL, top and bottom electrical contacts are typically provided above and below the active region, respectively, so that a pumping current can be applied through the active region. Because the optical gain is low in a vertical cavity design, the reflectors require a high reflectivity in order to achieve a sufficient level of feedback for the device to lase.
DBRs are typically formed of multiple pairs of layers referred to as mirror pairs. DBRs are sometimes referred to as mirror stacks. The pairs of layers are formed of a material system generally consisting of two materials having different indices of refraction and being easily lattice matched to the other portions of the VCSEL, to permit epitaxial fabrication techniques. The layers of the DBR are quarter-wave optical-thickness (QWOT) layers of alternating high and low refractive indices, where each mirror pair contains one high and one low refractive index QWOT layer. The number of mirror pairs per stack may range from 20-40 pairs to achieve a high percentage of reflectivity, depending on the difference between the refractive indices of the layers. A larger number of mirror pairs increases the percentage of reflected light (reflectivity).
The DBR mirrors of a typical VCSEL can be constructed from dielectric (insulating) or semiconductor layers (or a combination of both, including metal mirror sections). The difference in the refractive indices of adjacent layers in a DBR is referred to as the N-contrast from layer to layer. A lower N-contrast from layer to layer provides lower reflectivity, for a given number of mirror pairs. Semiconductor material typically has lower N-contrast than dielectric, and thus requires more layers for the same reflectivity. Thus, the difference between the refractive indices of the layers of the mirror pairs can be higher in dielectric DBRs, generally imparting higher reflectivity to dielectric DBRs than to semiconductor DBRs for the same number of mirror pairs and overall thickness. Conversely, in a dielectric DBR, a smaller number of mirror pairs can achieve the same reflectivity as a larger number in a semiconductor DBR. However, while a dielectric may function better as a DBR material in terms of reflectivity, it is not electrically conducting, which can be disadvantageous in some VCSEL applications. Thus, it is sometimes necessary or desirable to use semiconductor DBRs, despite their lower reflectivity/greater thickness, to conduct current, for example (e.g., in an EP VCSEL). Semiconductor DBRs also have higher thermal (heat) conductivity than do dielectric DBRs, for a given thickness (although semiconductor DBRs tend to be thicker, for a given reflectivity, thus reducing or possibly eliminating its heat conductivity advantage, in some designs).
When properly designed, these mirror pairs will cause a desired reflectivity at the laser wavelength. Typically in a VCSEL, the mirrors are designed so that the bottom DBR mirror (i.e. the one interposed between the substrate material and the active region) has nearly 100% reflectivity, while the top DBR mirror has a reflectivity that may be 98%-99.5% (depending on the details of the laser design). The partially reflective top (exit) mirror passes a portion of the coherent light built up in the resonating cavity formed by the active region and top and bottom mirrors. The optical path of the light is referred to as the effective cavity length, which is equal to the physical length of the cavity augmented by the phase penetration depths for the top and bottom DBR mirrors. VCSELs, DBRs, and related matters are discussed in further detail in Vertical-Cavity Surface-Emitting Lasers: Design, Fabrication, Characterization, and Applications, eds. Carl W. Wilmsen, Henryk Temkin and Larry A. Coldren (Cambridge: Cambridge University Press, 1999). DBR penetration depth and related matters, in particular, are discussed in this text at section 3.2.2.
DBR reflectivity is characterized by a complex amplitude and phase spectrum. The amplitude or reflectivity spectrum indicates how much reflectivity the DBR has as a function of wavelength of light reflected. The phase spectrum determines how much phase shift (change) the DBR will impart to reflected light (either at the penetration depth or at the surface of the DBR), again, as a function of wavelength of the light reflected. The lasing wavelength of a VCSEL is determined by the gain spectrum of the active region, the reflectivity of the mirrors, the effective (optical) length of the resonance cavity, and any phase shift imparted by the mirrors. The optical cavity length depends on the physical length or distance between the DBRs as well as reflectivity characteristics of the DBRs. Thus, the physical distance between the DBRs, as well as the phase characteristics of the DBRs, affect the lasing wavelength. Both the reflectivity spectrum and phase characteristics of a DBR can be varied by changing the structural characteristics of the DBR, i.e. the number, composition, and thicknesses of the layers of the DBR.
In standard VCSELs, the active region and top and bottom mirrors are monolithically fabricated on a substrate. A variant on the standard VCSEL, an external cavity VCSEL, or vertical-external-cavity surface-emitting laser (VECSEL), is also in use. In this case, the active region and bottom mirror are monolithically fabricated on a substrate, while the top mirror is mounted externally, some distance (typically very small) above the active region. The term VCSEL may be used herein to refer to both standard (monolithic) VCSELs and VECSELs.
Long wavelength (1.3 xcexcm to 1.55 xcexcm) VCSELs are of great interest in the optical telecommunications industry because of the minimum fiber dispersion at 1310 nm and the minimum fiber loss at 1.55 xcexcm (1550 nm).
VCSELs are used in a variety of applications. In telecommunications, for example, output laser light of a precise wavelength is modulated to encode and transmit information. The laser may be externally modulated, or directly modulated. A typical telecommunications system uses optical fiber to guide the radiation from the modulation (or emission) point to the detection point. When a fiber is used, it is necessary to couple the laser to the fiber, i.e. to couple the output laser light into the fiber end so that enough of the light is transmitted through the fiber core. The light output may be conditioned before it is coupled into a fiber; for example it may be amplified (e.g., by a semiconductor optical amplifier (SOA)) or modulated by a modulator. In this case, the laser must be coupled to the laser light-receiving or conditioning device, such as the SOA or modulator. Because the light conditioning device is coupled to the fiber, the laser is effectively coupled to the fiber by coupling it to the light conditioning device.
In various communicationsxe2x80x94e.g., telecommunicationsxe2x80x94applications, it is desirable that the emitted laser radiation of a given semiconductor laser have one of a number of precisely specified wavelengths, for example lasing wavelengths of 1310 nm (and other closely spaced wavelengths), or those specified by the ITU grid, such as lasing wavelengths of 1.55 xcexcm (and other closely spaced wavelengths). For example, the ITU grid specifies 45 standard ITU wavelengths for DWDM, 100 GHz spacing, to-wit, 1528.77 nm (196.1 THz), 1529.55 nm (196.0 THz), . . . , to 1563.86 nm (191.7 THz). This standard varies the optical frequency in 100 GHz (0.1 THz) increments. Other wavelength standards may also be employed, e.g. using a 200 GHz or 50 GHz channel spacing plan derivative of the 100 GHz ITU standard. ITU grid wavelengths are used in telecommunications applications such as coarse and dense wavelength-division multiplexing (CWDM and DWDM). In WDM, typically used in optical fiber communications, two or more optical (e.g. laser) signals having different wavelengths are simultaneously transmitted in the same direction over one fiber, and then are separated by wavelength at the distant end.
For this reason, it is desirable to be able to fabricate VCSELs of specified, different output wavelengths, or to fabricate tunable VCSELs having a given wavelength range, a particular wavelength of which may be selected in accordance with a selectable tuning parameter. For example, VCSELs can have a wavelength (within a given range) significantly dependent on drive current (or some other tuning parameter).